1. Technical Field
The present invention relates to optical communication systems and methods and, more particularly, systems and methods for coherent optical receivers in polarization multiplexed quadrature phase shift keyed systems.
2. Description of the Related Art
Continuous expansion of network applications results in a continuous demand on transmission capacities. To keep up with these demands, one can increase the transmission rates of the transceivers, or optimize the utilization of currently available components. In optical networks, increasing the transceivers' transmission rates augments the susceptibility of the transmitted signals to degradation over extended transmission lengths. This is due to the fact that, at higher data rates, the signal quality degrades severely as a result of linear and nonlinear impairments.
Chromatic dispersion (CD) and polarization mode dispersion (PMD) are the most dominant optical-channel distortion effects. CD is usually a much larger impairment than PMD, and can be a significant distortion even at relatively low data rates on long fibers.
CD is an effect based either in the refractivity of a medium or in the geometric properties of the medium, which cause different frequencies of electromagnetic radiation to travel through the medium at different rates. The result is that a pulse of light spreads out along the fiber as it travels over great distances. The longer the fiber over which a pulse travels, the wider the pulse spreads out. Difficulties arise when the resulting energy from a pulse begins to interfere with that of an adjacent pulse. This interference causes inter-symbol interference in the electrical domain.
CD effects are determined by each optical fiber, and can typically be considered stable over time. Because of its stability, CD can be compensated for using a passive device (e.g., medium having dispersion effects which counteract the dispersion of the transmission medium). However, such passive devices have drawbacks, in that they substantially reduce the optical signal-to-noise ratio.
PMD, meanwhile, is an effect based on the defects of the transmission medium and cannot be compensated for passively. In an ideal medium, signals traveling in orthogonal polarizations will travel at the same speed. In real media, however, defects cause random differences in the speeds of the respective polarizations, causing the polarizations to drift with respect to one another. PMD, in contrast to CD, is not a significant problem for most fibers until data rates exceed 10 Gb/s. However, in contrast to CD, PMD on long fibers changes randomly over time. The dynamic characteristic of PMD makes it a difficult problem for high-speed optical transmissions.
Polarization multiplexing (PolMux) with quadrature phase shift keying (QPSK) has been investigated as one avenue of research for boosting spectral efficiency and transmission rates. However, coherent detection in such systems has been considered too complex, due to phase- and polarization-tracking concerns, to be practical. Previous work has used digital filters with blind equalization to estimate transmission channels. Unfortunately, blind equalization lacks the reliability of a trained system. Conventional trained systems operate by time-multiplexing a training signal with data. This reduces the spectral efficiency.